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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::experimental::</span>is_detected, <span style="font-size:0.7em; line-height:130%">std::experimental::</span>detected_t, <span style="font-size:0.7em; line-height:130%">std::experimental::</span>detected_or</h1>
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<table class="t-dcl-begin"><tbody>
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<td> <div>定义于头文件 <code><a href="https://zh.cppreference.com/mwiki/index.php?title=cpp/header/experimental/type_traits&amp;action=edit&amp;redlink=1" class="new" title="cpp/header/experimental/type traits（页面不存在）">&lt;experimental/type_traits&gt;</a></code>
 </div></td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl t-since-libfund_ts_2">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">&gt;</span><br>
<span class="kw1">using</span> is_detected <span class="sy1">=</span> <span class="coMULTI">/* see below */</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span> </td>
</tr>
<tr class="t-dcl t-since-libfund_ts_2">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">&gt;</span><br>
<span class="kw1">using</span> detected_t <span class="sy1">=</span> <span class="coMULTI">/* see below */</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span> </td>
</tr>
<tr class="t-dcl t-since-libfund_ts_2">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> Default, <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">&gt;</span><br>
<span class="kw1">using</span> detected_or <span class="sy1">=</span> <span class="coMULTI">/* see below */</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span> </td>
</tr>

<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>别名模板 <code>detected_or</code> 是拥有二个公开成员 typedef  <code>value_t</code> 和 <code>type</code> 的未指定类类型的别名，它们定义如下：
</p>
<ul><li> 若<i>模板 id</i> <span class="t-c"><span class="mw-geshi cpp source-cpp">Op<span class="sy1">&lt;</span>Args...<span class="sy1">&gt;</span></span></span> 代表合法类型，则 <code>value_t</code> 是 <span class="t-lc"><a href="../types/integral_constant.html" title="cpp/types/integral constant">std::true_type</a></span> 的别名，而 <code>type</code> 是 <span class="t-c"><span class="mw-geshi cpp source-cpp">Op<span class="sy1">&lt;</span>Args...<span class="sy1">&gt;</span></span></span> 的别名；
</li><li> 否则， <code>value_t</code> 是 <span class="t-lc"><a href="../types/integral_constant.html" title="cpp/types/integral constant">std::false_type</a></span> 的别名而 <code>type</code> 是 <code>Default</code> 的别名。
</li></ul>
<p><br>
别名模板 <code>is_detected</code> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span> detected_or<span class="sy1">&lt;</span><a href="nonesuch.html"><span class="kw2414">std::<span class="me2">experimental</span><span class="sy4">::</span><span class="me2">nonesuch</span></span></a>, Op, Args...<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value_t</span></span></span> 。若<i>模板 id</i> <span class="t-c"><span class="mw-geshi cpp source-cpp">Op<span class="sy1">&lt;</span>Args...<span class="sy1">&gt;</span></span></span> 代表合法类型，则它是 <span class="t-lc"><a href="../types/integral_constant.html" title="cpp/types/integral constant">std::true_type</a></span> 的别名；否则它是 <span class="t-lc"><a href="../types/integral_constant.html" title="cpp/types/integral constant">std::false_type</a></span> 的别名。
</p><p><br>
别名模版 <code>detected_t</code> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span> detected_or<span class="sy1">&lt;</span><a href="nonesuch.html"><span class="kw2414">std::<span class="me2">experimental</span><span class="sy4">::</span><span class="me2">nonesuch</span></span></a>, Op, Args...<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span></span></span> 。若<i>模板 id</i> <span class="t-c"><span class="mw-geshi cpp source-cpp">Op<span class="sy1">&lt;</span>Args...<span class="sy1">&gt;</span></span></span> 代表合法类型，则它是 <span class="t-c"><span class="mw-geshi cpp source-cpp">Op<span class="sy1">&lt;</span>Args...<span class="sy1">&gt;</span></span></span> 的别名；否则它是类 <a href="nonesuch.html" title="cpp/experimental/nonesuch"><tt>std::experimental::nonesuch</tt></a> 的别名。
</p>
<h3><span class="mw-headline" id=".E9.99.84.E5.8A.A0.E5.B7.A5.E5.85.B7"> 附加工具 </span></h3>
<table class="t-dcl-begin"><tbody>
<tr class="t-dcl t-since-libfund_ts_2">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">&gt;</span><br>
<span class="kw4">constexpr</span> <span class="kw4">bool</span> is_detected_v <span class="sy1">=</span> is_detected<span class="sy1">&lt;</span>Op, Args...<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span> </td>
</tr>
<tr class="t-dcl t-since-libfund_ts_2">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">class</span> Default, <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">&gt;</span><br>
<span class="kw1">using</span> detected_or_t <span class="sy1">=</span> <span class="kw1">typename</span> detected_or<span class="sy1">&lt;</span>Default, Op, Args...<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span> </td>
</tr>
<tr class="t-dcl t-since-libfund_ts_2">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> Expected, <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">&gt;</span><br>
<span class="kw1">using</span> is_detected_exact <span class="sy1">=</span> is_same<span class="sy1">&lt;</span>Expected, detected_t<span class="sy1">&lt;</span>Op, Args...<span class="sy1">&gt;&gt;</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span> </td>
</tr>
<tr class="t-dcl t-since-libfund_ts_2">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> Expected, <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">&gt;</span><br>
<p><span class="kw4">constexpr</span> <span class="kw4">bool</span> is_detected_exact_v <span class="sy1">=</span><br>
</p>
    is_detected_exact<span class="sy1">&lt;</span>Expected, Op, Args...<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span> </td>
</tr>
<tr class="t-dcl t-since-libfund_ts_2">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> To, <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">&gt;</span><br>
<span class="kw1">using</span> is_detected_convertible <span class="sy1">=</span> is_convertible<span class="sy1">&lt;</span>detected_t<span class="sy1">&lt;</span>Op, Args...<span class="sy1">&gt;</span>, To<span class="sy1">&gt;</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span> </td>
</tr>
<tr class="t-dcl t-since-libfund_ts_2">
<td> <div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> To, <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">&gt;</span><br>
<p><span class="kw4">constexpr</span> <span class="kw4">bool</span> is_detected_convertible_v <span class="sy1">=</span><br>
</p>
    is_detected_convertible<span class="sy1">&lt;</span>To, Op, Args...<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span><span class="sy4">;</span></span></div></td>
<td class="t-dcl-nopad">  </td>
<td> <span class="t-mark-rev t-since-libfund-ts-2 t-mark-ts">(库基础 TS v2)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<p>别名模版 <code>is_detected_exact</code> 检查 <span class="t-c"><span class="mw-geshi cpp source-cpp">detected_t<span class="sy1">&lt;</span>Op, Args...<span class="sy1">&gt;</span></span></span> 是否为 <code>Expected</code> 。
</p><p>别名模板 <code>is_detected_convertible</code> 检查 <span class="t-c"><span class="mw-geshi cpp source-cpp">detected_t<span class="sy1">&lt;</span>Op, Args...<span class="sy1">&gt;</span></span></span> 是否可转换为 <code>To</code> 。
</p>
<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0"> 可能的实现 </span></h3>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">namespace</span> detail <span class="br0">{</span>
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> Default, <span class="kw1">class</span> AlwaysVoid,
          <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw1">struct</span> detector <span class="br0">{</span>
  <span class="kw1">using</span> value_t <span class="sy1">=</span> <a href="../types/integral_constant.html"><span class="kw650">std::<span class="me2">false_type</span></span></a><span class="sy4">;</span>
  <span class="kw1">using</span> type <span class="sy1">=</span> Default<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> Default, <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw1">struct</span> detector<span class="sy1">&lt;</span>Default, <a href="../types/void_t.html"><span class="kw632">std::<span class="me2">void_t</span></span></a><span class="sy1">&lt;</span>Op<span class="sy1">&lt;</span>Args...<span class="sy1">&gt;&gt;</span>, Op, Args...<span class="sy1">&gt;</span> <span class="br0">{</span>
  <span class="co1">// 注意 std::void_t 是 C++17 特性</span>
  <span class="kw1">using</span> value_t <span class="sy1">=</span> <a href="../types/integral_constant.html"><span class="kw649">std::<span class="me2">true_type</span></span></a><span class="sy4">;</span>
  <span class="kw1">using</span> type <span class="sy1">=</span> Op<span class="sy1">&lt;</span>Args...<span class="sy1">&gt;</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="br0">}</span> <span class="co1">// namespace detail</span>
 
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw1">using</span> is_detected <span class="sy1">=</span> <span class="kw1">typename</span> detail<span class="sy4">::</span><span class="me2">detector</span><span class="sy1">&lt;</span>nonesuch, <span class="kw4">void</span>, Op, Args...<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value_t</span><span class="sy4">;</span>
 
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw1">using</span> detected_t <span class="sy1">=</span> <span class="kw1">typename</span> detail<span class="sy4">::</span><span class="me2">detector</span><span class="sy1">&lt;</span>nonesuch, <span class="kw4">void</span>, Op, Args...<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">type</span><span class="sy4">;</span>
 
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> Default, <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span>...<span class="sy1">&gt;</span> <span class="kw1">class</span> Op, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">&gt;</span>
<span class="kw1">using</span> detected_or <span class="sy1">=</span> detail<span class="sy4">::</span><span class="me2">detector</span><span class="sy1">&lt;</span>Default, <span class="kw4">void</span>, Op, Args...<span class="sy1">&gt;</span><span class="sy4">;</span></pre></div></div>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;experimental/type_traits&gt;</span>
<span class="co2">#include &lt;cstddef&gt;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
<span class="kw1">using</span> copy_assign_t <span class="sy1">=</span> decltype<span class="br0">(</span><a href="http://zh.cppreference.com/w/cpp/utility/declval"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1">&lt;</span>T<span class="sy3">&amp;</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <a href="http://zh.cppreference.com/w/cpp/utility/declval"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1">&lt;</span><span class="kw4">const</span> T<span class="sy3">&amp;</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> Meow <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> Purr <span class="br0">{</span> <span class="kw4">void</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> Purr<span class="sy3">&amp;</span><span class="br0">)</span> <span class="sy1">=</span> delete<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
static_assert<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">experimental</span><span class="sy4">::</span><span class="me2">is_detected</span><span class="sy1">&lt;</span>copy_assign_t, Meow<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span>,
              <span class="st0">"Meow should be copy assignable!"</span><span class="br0">)</span><span class="sy4">;</span>
static_assert<span class="br0">(</span><span class="sy3">!</span>std<span class="sy4">::</span><span class="me2">experimental</span><span class="sy4">::</span><span class="me2">is_detected_v</span><span class="sy1">&lt;</span>copy_assign_t, Purr<span class="sy1">&gt;</span>,
              <span class="st0">"Purr should not be copy assignable!"</span><span class="br0">)</span><span class="sy4">;</span>
static_assert<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">experimental</span><span class="sy4">::</span><span class="me2">is_detected_exact_v</span><span class="sy1">&lt;</span>Meow<span class="sy3">&amp;</span>, copy_assign_t, Meow<span class="sy1">&gt;</span>,
              <span class="st0">"Copy assignment of Meow should return Meow&amp;!"</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
<span class="kw1">using</span> diff_t <span class="sy1">=</span> <span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">difference_type</span><span class="sy4">;</span>
 
<span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> Ptr<span class="sy1">&gt;</span>
<span class="kw1">using</span> difference_type <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">experimental</span><span class="sy4">::</span><span class="me2">detected_or_t</span><span class="sy1">&lt;</span><a href="../types/ptrdiff_t.html"><span class="kw101">std::<span class="me2">ptrdiff_t</span></span></a>, diff_t, Ptr<span class="sy1">&gt;</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> Woof <span class="br0">{</span> <span class="kw1">using</span> difference_type <span class="sy1">=</span> <span class="kw4">int</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> Bark <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
 
static_assert<span class="br0">(</span><a href="../types/is_same.html"><span class="kw588">std::<span class="me2">is_same</span></span></a><span class="sy1">&lt;</span>difference_type<span class="sy1">&lt;</span>Woof<span class="sy1">&gt;</span>, <span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span>,
              <span class="st0">"Woof's difference_type should be int!"</span><span class="br0">)</span><span class="sy4">;</span>
static_assert<span class="br0">(</span><a href="../types/is_same.html"><span class="kw588">std::<span class="me2">is_same</span></span></a><span class="sy1">&lt;</span>difference_type<span class="sy1">&lt;</span>Bark<span class="sy1">&gt;</span>, <a href="../types/ptrdiff_t.html"><span class="kw101">std::<span class="me2">ptrdiff_t</span></span></a><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span>,
              <span class="st0">"Bark's difference_type should be ptrdiff_t!"</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span></pre></div></div>
<p><br> 
</p>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>

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